1. Field of the Invention
The invention relates to methods for removing gases from liquid metal, and more specifically to two preferred embodiments relating to methods for removing dissolved hydrogen gas from liquid aluminum.
2. Description of the Related Art
During manufacture of aluminum, it is necessary to remove hydrogen when the aluminum is in a liquid state in order to minimize the porosity of the resulting cast product. By one conventional method, an inert carrier gas is directed through the melt to mix with and remove dissolved hydrogen gas. Conventional devices use an argon carrier gas which is injected into the melt, causing hydrogen to enter the gas bubble and escape with the argon. This gas is then exhausted into the environment. Substantial cost is involved with removing hydrogen due to the significant volume of argon required to remove a relatively small volume of hydrogen, and the product gas is exhausted into the environment. It would be desirable to retain and recycle the inert gas which is otherwise lost.
In another conventional process, an inert carrier gas such as argon or nitrogen is injected into the melt with a small amount of chlorine gas. Chlorine reacts with undesirable contents of dissolved alkaline and alkaline earth metals, such as sodium and calcium, to form salts which are fluxed out of the melt. Some of the chlorine reacts with hydrogen dissolved in the melt to form HCl, since HCl is stable. However, some excess chlorine also escapes to the environment. Chlorine gas is costly and difficult to handle, and it is undesirable to release chlorine and HCl into the ambient. This is due to its deleterious effect to the environment, and its corrosive effects upon the surrounding equipment and structures. It would be desirable to capture and retain the chlorine within a closed system and to recycle this part of the chlorine which is used in melt treatment systems.